A Comparative Analysis of Different Control Strategies for Three-Level Neutral Point Clamped based Photovoltaic Central Inverter under Unbalanced Grid Operation

This paper investigates the different control techniques need to be applied to a three-phase three-level neutral point clamped based photovoltaic central inverter connected to an unbalanced utility grid. The power conversion stage for integrating the PV source to three-phase utility grid consists of a three-level DC-DC boost converter + three-phase three-level NPC based central inverter. The three-level boost converter helps in balancing the individual DC link capacitor voltages during sub-MPPT mode of operation. With unbalanced grid operation, the central inverter has been controlled to maintain either the balanced positive sequence components of three-phase grid current or a reduction in grid side active and reactive power fluctuations by allowing a certain amount of negative sequence current in the system. The entire system has been simulated in PLECS simulation tool to obtain the power loss distribution incurred in different parts of the system. This paper analyses the different power losses incurred in system for different control strategies when the PV central inverter injects power to an unbalanced utility grid and compares them to obtain the most efficient one.